1. Field of the Invention
This invention relates to a method for milling a test slice, and more particularly to a method of milling a transmission electron microscope (TEM) test slice, which has a uniform thickness.
2. Description of Related Art
In a failure analysis of a vary-large-scale-integration (VLSI) device, the analysis of a cross-sectional surface has been accepted as an effective diagnostic technology. A scanning electron microscope (SEM) is one of the diagnostic tools used to observe the cross-sectional surface but its resolution is poor when integration density is high, as is the case with the VLSI device. The TEM is another diagnostic tool, which has been introduced to gradually replace the SEM. In order to maintain the reliability of the VLSI device and streamline its production, the TEM is becoming more commonly used for damage analyses of VLSI devices.
Because of the application of the TEM, the production of a test slice with an allowed thickness becomes an important issue. Generally, the test slice is kept at a thickness less than 0.25 micron so that electrons from the TEM can travel through it. In light of these considerations, the focused ion beam (FIB) was developed to mill the test slice to a predetermined thickness.
FIG. 1 is a schematic perspective view of a test slice. A whole slice is cut out from a predetermined position and is then re-cut to produce a test slice 100, of a given dimension, as shown in FIG. 1. The test slice 100 has a thickness about 25-35 microns. One example is 30 microns. Using a FIB 102 to mill the test slice 100 from the top and gradually reach the dotted side surface forms a TEM observable wall 104. The thickness of the TEM observable wall 104 should be less than about 0.25 microns as required by a TEM 106, which is applied to diagnose the TEM observable wall 104.
In FIG. 2A, the upper plot is a schematic top view of the test slice in FIG. 1 showing the structure of the test slice 100. The lower plot is a schematic cross-sectional view taken along the line I--I in the upper plot. The shaded region is the region milled by the FIB 102. The TEM observable wall 104 has a thickness "d.sub.1 ", which is less than about 0.25 microns. The method described above is called the one-sided method.
Another method is called the two-sided method, and is similar to the one-sided method. The difference is that, in the two-sided method, both sides are milled as shown in FIG. 2B. In FIG. 2B, the upper plot is a schematic top view of another test slice; the lower plot is a schematic cross-sectional view taken along the line II--II in the upper plot. A TEM observable wall is formed at the middle of the test slice with a thickness of "d.sub.1 " less than about 0.25 micron.
FIG. 3A is a top view schematically illustrating the conventional milling flow of a TEM test slice. FIG. 3B is a schematic cross-sectional view taken along the III--III line on the FIG. 3A.
In FIG. 3A and FIG. 3B, A1, A2, A3, and A4 correspond to B1, B2, B3, and B4, respectively. In a step A1, a sacrificial layer 310, indicated as a shaded region, is coated on the top surface of a TEM test slice 300, which is shown as the square. The sacrificial layer 310 made of, for example, platinum (Pt) serves as a milling stop signal. In a next step A2, a FIB with a large current roughly mills the TEM test slice. A milling front 320a indicates the place where the FIB milling process has occurred. In a next step A3, a FIB with middle current mills the TEM test slice. A milling front 320b indicates accomplishment of another milling step, which has reached the sacrificial layer 310. Then, in a next step A4, a FIB with small current mills the TEM test slice. A milling front 320c is further formed. The region under the sacrificial layer 310 is a TEM observable wall, which is not uniform in thickness.
FIG. 4 is a schematic drawing of a cross-sectional view in detail, according to FIG. 3B. In FIG. 4, a shaded region represents the sacrificial layer 310 shown in FIG. 3B. A slanted surface 410 of the TEM observable wall is automatically formed due to FIB milling. The slanted surface 410 has a slanting angle .theta. away from a normal line, which is taken along a normal direction of the top surface of the TEM test slice. The normal line, in fact, is also along a FIB incident direction. The thickness, also called the width, of the TEM observable wall is gradually wider from the top to the bottom. Since the thickness is progressively wider, the thickness around the bottom region may be too thick even though the top region has a suitable thickness. If the thickness is too thick, TEM can not effectively penetrate the TEM observable wall, and then TEM diagnosis is therefore limited only on the top region.
Moreover, the desired thickness of the TEM observable wall is very small, for example, 0.1 microns. Moreover, the desired thickness is based on the milling stop signal, which depends on the residual sacrificial layer 310 shown in FIG. 3A and FIG. 3B after milling. This thickness of 0.1 microns cannot be easily controlled. If over-milling occurs, the test slice is then damaged. On the contrary, if insufficient milling occurs, the test slice is then not suitable for TEM diagnoses. It becomes an important issue to choose a proper type of milling stop signal in order to obtain a TEM test slice with a proper uniform thickness.